Remove den._.bidirectional

[vic]

NOTE: for the time being, I've added asides to https://den.oeiuwq.com/guides/bidirectional/

Removing den._.bidirectional from core was a very good move. It helped me realize that bi-directionality was causing more confusion to people due to duplicates than solutions. I believe this bidir being in core from the start was what caused all those duplicate-definitions problems we have seen so far. So moving away from it as much as possible is good.

Then, after talking with @drupol about it, I've been thinking that maybe we are much better without den._.bidirectional.

Our bidirectional page, now recommends not to use it, but instead share a common aspect between users, this common aspect can contribute nixos, homeManager or anything else without having to re-active the host aspect (and thus causing dups).

I did a grep on my own infra rg bidirectional and noticed I'm not even using it!, because in my infra all I needed was "vic user contributes to all hosts" and NOT "host contributes to all users".

So I'm wondering now, why my mind played that trick on me thinking that bidirectionaly was needed right from the start, and it was the cause of so many bugs.

Den is just starting life but it is now being used by more people, maybe not all of them know that bidirectionality even existed, if so I'd love to remove it.

cc @ProducerMatt and maybe many others that have hit duplicate-definition errors.

[vic]

Some details

without bidir enabled

# host aspect and its includes only care about OS level classes
den.aspects.igloo = {
  nixos = ...;
  includes = [ foo bar ];
};

# user aspect and its includes can produce `nixos`, `homeManager`, etc.
# this is because `homeManager` is just a forwarding into nixos class.
den.aspects.tux = {
  nixos = ...;
  homeManager = ...;
  includes = [ moo mar ];
}

with bidir enabled

# host and its includes **also** configure home-envs.
# the problem is that the host-aspect is re-activated by bidirectional,
# ctx pipeline as documented: igloo{host} -> tux{host,user} -> igloo{host,user}
den.aspects.igloo = {
  nixos = ...;
  homeManager = ...;
};

[HeitorAugustoLN]

To be honest, bidirectional is really confusing to me, like I don't use it, but I do have aspects on that I include in my host aspect that are for all users, so I don't really get the difference. Though I really prefer the mutual provider approach